1. Field of the Invention
The present invention relates to a swash plate servo device. More particularly, the present invention relates to a swash plate servo device for tilting and rolling a swash plate in a swash plate plunger type hydraulic unit such as a swash plate plunger pump or swash plate plunger motor.
2. Description of the Background Art
Heretofore, various types of hydraulic continuously variable transmissions each comprising a combination of a hydraulic pump and a hydraulic motor have been known and implemented. For example, hydraulic continuously variable transmissions disclosed in JP-A 6-2753 and JP-B 7-88884 are proposed by the applicant of the present application. Each of the hydraulic continuously variable transmissions disclosed by these patent documents comprises a swash plate plunger pump, a swash plate plunger motor and a hydraulic closing circuit for connecting the outlet port and inlet port of the swash plate plunger pump to the inlet port and outlet port of the swash plate plunger motor, respectively, wherein a pump swash plate member is driven by an engine, a pump cylinder and a motor cylinder are connected to each other above the output shaft of the engine, the rotation of a motor swash plate member is restricted, and the angle of a motor swash plate can be changed.
In this hydraulic continuously variable transmission, the control of non-stage speed change for changing the output revolution of the motor continuously is carried out by altering the angle of the motor swash plate to change the capacity of the motor. The angle of the motor swash plate can be changed by converting the revolution of a swash plate control motor (electric motor) external to the housing of the transmission into linear motion by means of a ball screw mechanism (revolution/linear motion conversion mechanism) to roll a motor rolling member (motor swash plate) (refer to JP-A 2001-343060, for example). This ball screw mechanism has a screw shaft turned by a swash plate control motor, and a nut member which is screwed to this screw shaft and connected to the end portion of the motor rolling member. A stopper is provided at both ends of the screw shaft to be contacted to the nut member in order to limit the moving range of the nut member.
The nut member constituting the ball screw mechanism has a ball thread holding a large number of balls by a cage on the inner wall of a nut body, and the side end portion of the nut body is contacted to the stopper to limit the moving range of the nut member. Since the ball thread is formed on the inner side of the side end portion of the nut body, when the side end portion of the nut body is contacted to the stopper, the ball thread is mated with the male screw of the screw shaft on the inner side of the stopper. Therefore, the outer end portion of the male screw does not contribute to this mating and is of no use. Therefore, if this male screw portion which does not contribute to mating is eliminated, the male screw can be used more effectively and the processing cost can be reduced. In other words, if a portion which is not used for mating with the nut member due to its contact to the stopper can also be used for mating with the nut member, the moving range of the nut member can be increased with the male screw of the known art.